[IMP] mass_mailing: add preview block for mailings

PURPOSE

When displaying an email in a list the mail client (gmail, outlook...) computes
a preview based on the content. This preview is generally not well computed as
it contains a lot of garbage and tags while it should contain only relevant
text.

SPECIFICATIONS

To build this preview, all mail clients read the content of the email.
The only way to be able to customize the preview is to add an invisible
HTML element at the beginning of the email with the wanted preview text.

We add at the end of the preview `‌` (zero-width non-joiner) to
fill the end of the preview in order to not have the beginning of the
mail at the end of the preview. It doesn't work with simple space as
the mail clients trim each HTML element content.

Task ID-2172125
PR #49886

2 files changed, 67 insertions, 0 deletions

diff --git a/odoo/addons/base/tests/test_mail.py b/odoo/addons/base/tests/test_mail.py
index 5973fd623fb..69ad772447e 100644
--- a/odoo/addons/base/tests/test_mail.py
+++ b/odoo/addons/base/tests/test_mail.py
@@ -5,6 +5,7 @@
 from unittest.mock import patch
 import email.policy
 import email.message
+import re
 import threading
 
 from odoo.tests.common import BaseCase, SavepointCase, TransactionCase
@@ -12,6 +13,7 @@ from odoo.tools import (
     is_html_empty, html_sanitize, append_content_to_html, plaintext2html,
     email_split,
     misc, formataddr,
+    prepend_html_content,
 )
 
 from . import test_mail_examples
@@ -334,6 +336,57 @@ class TestHtmlTools(BaseCase):
         for content in valid_html_samples:
             self.assertFalse(is_html_empty(content))
 
+    def test_prepend_html_content(self):
+        body = """
+            <html>
+                <body>
+                    <div>test</div>
+                </body>
+            </html>
+        """
+
+        content = "<span>content</span>"
+
+        result = prepend_html_content(body, content)
+        result = re.sub(r'[\s\t]', '', result)
+        self.assertEqual(result, "<html><body><span>content</span><div>test</div></body></html>")
+
+        body = "<div>test</div>"
+        content = "<span>content</span>"
+
+        result = prepend_html_content(body, content)
+        result = re.sub(r'[\s\t]', '', result)
+        self.assertEqual(result, "<span>content</span><div>test</div>")
+
+        body = """
+            <body>
+                <div>test</div>
+            </body>
+        """
+
+        result = prepend_html_content(body, content)
+        result = re.sub(r'[\s\t]', '', result)
+        self.assertEqual(result, "<body><span>content</span><div>test</div></body>")
+
+        body = """
+            <html>
+                <body>
+                    <div>test</div>
+                </body>
+            </html>
+        """
+
+        content = """
+            <html>
+                <body>
+                    <div>test</div>
+                </body>
+            </html>
+        """
+        result = prepend_html_content(body, content)
+        result = re.sub(r'[\s\t]', '', result)
+        self.assertEqual(result, "<html><body><div>test</div><div>test</div></body></html>")
+
 
 class TestEmailTools(BaseCase):
     """ Test some of our generic utility functions for emails """
diff --git a/odoo/tools/mail.py b/odoo/tools/mail.py
index eed992e142f..c98a575f10a 100644
--- a/odoo/tools/mail.py
+++ b/odoo/tools/mail.py
@@ -428,6 +428,20 @@ def append_content_to_html(html, content, plaintext=True, preserve=False, contai
         return '%s%s' % (html, content)
     return '%s%s%s' % (html[:insert_location], content, html[insert_location:])
 
+
+def prepend_html_content(html_body, html_content):
+    """Prepend some HTML content at the beginning of an other HTML content."""
+    html_content = re.sub(r'(?i)(</?(?:html|body|head|!\s*DOCTYPE)[^>]*>)', '', html_content)
+    html_content = html_content.strip()
+
+    insert_index = next(re.finditer(r'<body[^>]*>', html_body), None)
+    if insert_index is None:
+        insert_index = next(re.finditer(r'<html[^>]*>', html_body), None)
+
+    insert_index = insert_index.end() if insert_index else 0
+
+    return html_body[:insert_index] + html_content + html_body[insert_index:]
+
 #----------------------------------------------------------
 # Emails
 #----------------------------------------------------------